I'll offer a quick run down on plates. First you'll want to buy reference books but to start the seller mentions they are surplus pieces. The way they look is not suspect knowing that they were not issued or dug. You can still find unissued plates wrapped in their original paper and they are still shiny! The important things to check are of course a long list but look at the size, shape, design, color, fasteners and fill. Different makers have slightly different shapes in the plate or their lettering. Some US are fat lettered and some have skinny letters. Some SNY plates share these traits too. The fasteners on the back will be what collectors call puppy paw or arrowhead for most belt plates. Think of a dog begging and picture his paws hanging. The arrowheads are of course shaped like arrows but they should not be sharp in their edges. Look for soft beveled edges or a slight upward curve to them. Some early plates have the wire hooks like the NCO plate. Remember that breast plate design is from 1826. The loops should be iron and not something like stainless steel, the distance between loops varies but they should be short and fairly rounded on box plates. The lead fill should not exceed the edge of the plate and in many cases be concave. Some have the makers mark stamped into the back but many do not.